The present invention relates generally to certain aspects of a catalyst regenerator apparatus in a fluidized catalytic cracking unit. In other aspects, this invention relates to improved methods of and apparatus for the regeneration of catalytic cracking catalyst.
In the catalytic cracking process, high boiling range petroleum fractions, which principally comprise high molecular weight hydrocarbons, are broken up or cracked to form lower boiling range fractions comprising lower molecular weight saturated or unsaturated hydrocarbons. The apparatus for practicing this process, in essence, comprises a reactor, a regenerator, a catalyst circulation system, and a fractionation system. Feedstock is injected into a riser line where it contacts and mixes with hot, freshly regenerated catalyst and passes to the reactor. Cracking takes place in the riser and in the reactor. In many catalytic cracking designs, the cracking reactions take place in a single riser-reactor which serves as the reactor.
As a result of the cracking, the catalyst becomes coked with carbonaceous deposits and loses much of its activity. In order to restore the lost catalyst activity, the spent catalyst is transferred, often pneumatically or by gravity, to the regenerator where it is fluidized and where the coke deposits are burned off under carefully controlled operating conditions.
In the regenerator vessel to which the spent catalyst is transferred, oxygen-containing combustion gas, which generally, but not necessarily, is atmospheric air, is introduced by way of a gas distribution apparatus. This combustion gas is used to fluidize the catalyst and to provide oxygen for the burning of coke off the spent cracking catalyst thereby restoring its activity. The combustion gas can be supplied by numerous means such as, for example, perforated grid plates, pipe grids, single and multiple concentric perforated rings, and christmas trees. An important function of the gas distribution apparatus is to provide an uniform distribution of combustion gas having the appropriate flow conditions, which include such factors as proper velocity and energy profiles and orifice or nozzle pressure drop, for effective fluidization and regeneration of the spent cracking catalyst.
As a part of the gas distribution apparatus, there are often incorporated nozzles or nipples by which gas is discharged into the fluidized catalyst bed of the regenerator. Because of the abrasive action of the fluidized catalyst and high velocity flow of combustion gas within the regenerator, operators of catalytic cracking units commonly encounter severe erosion of the air distribution apparatus and the attaching parts which include the nozzles and nipples. A problem that often occurs, and which is addressed by the present invention, is the severe erosion of the gas nozzles or nipples caused by the impact of fluidized catalyst against their outside surfaces. A result that can occur from excessive nozzle erosion is inefficient catalyst regeneration caused by an upsetting and alteration of air distribution profile within the regenerator. In certain circumstances, the erosion can be so severe that premature shutdowns of the catalytic cracker or shorter time intervals between maintenance shutdowns are caused resulting in loss of production and increased maintenance costs. By reducing the rate of nozzle erosion, the useful life of the gas nozzles can be increased thereby providing for lower replacement costs and potentially longer intervals between maintenance shutdowns. A further benefit from the reduction of gas nozzle erosion is a decrease in catalyst attrition losses.